In the mammalian brain, like the pituitary, three immunoreactive prolactin (IR-PRL) like proteins have been identified with molecular weights of 24, 16, and 8K. In the rat hypothalamus the 24K brain PRL is physicochemically similar to that of the 24K pituitary PRL, and is not dependent of pituitary PRL. Estrogen is well known to increase pituitary PRL content. Interestingly, estrogen increases the concentration of hypothalamic IR-PRL in the female hypophysectomized rat. These studies are exciting because they demonstrate that a protein which is similar to pituitary PRL is localized in the brain, and its concentration is regulated by an estrogen dependent mechanism which is dependent of pituitary PRL. Further studies will be performed to determine if other factors which are known to alter pituitary PRL content will also affect brain PRL concentrations in hypophysectomized rats. The studies in this grant are also designed to learn if estrogen regulates the synthesis and/or degradation of IR-PRL in the rat hypothalamus, and to determine if IR-PRL in other brain regions is regulated by estrogen. In these studies gel electrophoresis and western blot analyses will be used to determine if estrogen regulates the concentrations of the three IR-PRL proteins. In addition, the brain can incorporate (35S)-methionine into a immunoprecipitable PRL like protein with a molecular weight of 24K. Peptide mapping experiments will be performed to determine if this material is similar to that of pituitary PRL. Studies will also be performed to determine if estrogen, or other factors regulate the rate of synthesis of the PRL-like protein in the rat brain. Finally, the rat brain has the capacity to proteolytically modify pituitary PRL. Studies will be performed to further characterize the metabolism of PRL in the brain, and to determine if the proteolytic activity of the brain is affected by estrogen, or other factors known to regulate pituitary PRL. These studies will allow for a better understanding of the relationship between proteins first identified in the anterior pituitary and brain function.